Conventional dual slope integrator analog to digital converters are generally preferred where low voltage input signals are to be measured. The dual slope technique provides excellent noise rejection and an accuracy that, in general, is independent of the clock pulse rate and integrating resistors and capacitors. However, such known circuits have a first order of sensitivity to zero drift and a second order of sensitivity to span error with temperature. Further, it is generally difficult to obtain bi-polar operation using conventional known techniques.
In its practical form, the accuracy of an analog to digital converter depends upon the following characteristics of the basic circuit:
(1) offset voltage of the pre-amplifier or buffer, integrator and cross-over detector;
(2) offset voltage drift of the pre-amplifier or buffer, integrator and cross-over detector;
(3) dynamics of the switches, capacitors, pre-amplifier, integrator and cross-over detector;
(4) discontinuities at or near zero signal associated with achieving bi-polar signal conversion;
(5) internal circuit noise;
(6) linearity of the amplifier, integrator and cross-over detectors.
Any analog to digital conversion circuit which would provide for improvement in the foregoing characteristics would be highly desirable.
In my U.S. Pat. No. 4,107,618 there is described an operational differential amplifier system wherein the signal gain of an input analog signal can be made independent of a reference voltage. The technique described in this patent can be used to advantage in the provision of a greatly improved analog to digital converter wherein zero drift is essentially eliminated.